Film for use in intermediate image transfer member and method of producing endless-belt-shaped film

ABSTRACT

A film is made of an insulating matrix resin and an electroconductive material, in which the electroconductive material is dispersed in the insulating matrix resin in such a manner that a surface resistivity of A [Ω] of the film and a volume resistivity of B [Ω·cm] of the film in the direction of a thickness thereof normal to the surface of the film satisfy a relationship of: A&gt;B, and when the film has a thickness of T [cm], the electroconductive material is dispersed in the insulating matrix resin in such a manner that the surface resistivity of A [Ω] of the film and the volume resistivity of B [Ω·cm] of the film satisfy a relationship of; A [Ω]×T [cm]&gt;B [Ω·cm]. This film can be prepared by a centrifugal molding method.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a film adopted for use in anintermediate image transfer member, which may be used in the form of anendless belt, to which a toner image can be transferred, and which iscapable of reducing the formation of image transfer dust and thedeposition of toner on the background of images in an image formationprocess.

[0003] The present invention also relates to an endless-belt-shaped filmmade of the above-mentioned film for use in the intermediate imagetransfer member.

[0004] 2. Discussion of Background

[0005] Conventionally, there is known an image formation apparatus inwhich a latent electrostatic image is formed on a latent image bearingmember such as a photoconductor drum, the latent electrostatic image isdeveloped with toner to a visible toner image, the visible toner imageis then electrostatically transferred to an endless-belt-shapedintermediate image transfer belt, and the toner image is furthertransferred from the intermediate image transfer belt to a recordingmaterial such as a transfer sheet.

[0006] In this kind of image formation apparatus, an electroconductiveintermediate image transfer belt is widely used as the above-mentionedintermediate image transfer belt.

[0007] However, it is difficult to set the surface resistivity or theelectroconductive intermediate image transfer belt at an appropriatevalue. For example, in Japanese Patent 2560727, it is described thatproper images can be formed when the surface resistivity of theelectroconductive image transfer image transfer belt is in a range of10⁷ to 10¹⁵ [Ω/□]. However, there is a case where the deposition oftoner on the background of images occurs and image transfer dust isformed, with a toner being transferred away from its right imagetransfer position even when the surface resistivity of theelectroconductive image transfer belt is set in the above-mentionedrange. It is also known that such image transfer dust occurs frequentlywhen an intermediate image transfer belt with lower surface resistivityis used. Such image transfer dust is considered to be caused by theelectric lines of force, formed between the intermediate image transferbelt and the latent electrostatic image bearing member, being disturbedin the direction of the surface of the intermediate image transfer belt.Furthermore, it is known that the deposition of toner on the backgroundof images occurs frequently when an intermediate image transfer beltwith higher surface resistivity is used. Thus, there is a dilemma inwhich when the surface resistivity of the intermediate image transferbelt is increased in order to reduce the formation of the image transferdust, the deposition of toner on the background of images is increased,while when the surface resistivity of the intermediate image transferbelt is decreased in order to reduce the deposition of toner on thebackground of images, the occurrence of the image transfer dust ispromoted.

[0008] In order to get out from such a dilemma, it will be necessary toset the surface resistivity of the intermediate image transfer belt at ahigh surface resistivity and also to provide a charge quenching unit forquenching electric charges of the intermediate image transfer belt.However, the provision of such a charge quenching unit not only makesthe image formation apparatus costly, but also makes the mechanism ofthe image formation apparatus complex. Therefore, the provision of thequenching apparatus is not a preferable step from the above point ofview. Furthermore, a corona charger, which is one of the simplestquenching apparatus, is considered to be usable for the above-mentionedpurpose. However, the corona charge has a shortcoming that it generatesozone and causes an air pollution problem.

SUMMARY OF THE INVENTION

[0009] It is therefore a first object of the present invention toprovide a film with a single layer structure, which can be adopted foruse in an intermediate image transfer member, and may be used in theform of an endless belt, to which a toner image can be transferred, andwhich is capable of reducing the formation of image transfer dust andthe deposition of toner on the background of images in an imageformation process, without the necessity for the provision of any chargequenching apparatus.

[0010] A second object of the present invention is to provide a methodof producing an endless-belt-shaped film made of the above-mentionedfilm for use in the intermediate image transfer member.

[0011] The first object of the present invention can be achieved by afilm which comprises an insulating matrix resin and an electroconductivematerial, in which the electroconductive material is dispersed in theinsulating matrix resin in such a manner that a surface resistivity of A[Ω] of the film and a volume resistivity of B [Ω·cm] of the film in thedirection of a thickness thereof normal to the surface of the filmsatisfy a relationship of:

[0012] A>B.

[0013] The first object of the present invention can also be achieved bya film comprising an insulating matrix resin and an electroconductivematerial, having a thickness of T [cm], the electroconductive materialbeing dispersed in the insulating matrix resin in such a manner that asurface resistivity of A [Ω9 of the film and a volume resistivity of B[Ω·cm] of the film in the direction of a thickness thereof normal to thesurface of the film satisfy a relationship of:

[0014] A [Ω]×T [cm]>B [Ω·cm].

[0015] In any of the above films, the electroconductive material maycomprise a first electroconductive material and a secondelectroconductive material, each of which has a different resistivity.

[0016] Furthermore, in any of the above films, the electroconductivematerial may comprise a first electroconductive material and a secondelectroconductive material, each of which has a different particle size.

[0017] Furthermore, in any of the above films, the electroconductivematerial may be in the shape of needles.

[0018] Each of the needles of the electroconductive material may be insuch a shape that the thickness thereof is decreased toward opposite endportions thereof.

[0019] As the above-mentioned electroconductive material, anelectroconductive material comprising carbon can be used.

[0020] The first object of the present invention can also be achieved bya film with a single layer structure comprising a first region extendingalong a surface of the film and a second region extending under thefirst region, the first region comprising an insulting matrix resin anda first electroconductive material dispersed in the form of particles inthe insulating matrix resin, and the second region comprising theinsulating matrix resin and a second electroconductive materialdispersed in the form of particles in the insulating matrix resin, thefirst electroconductive material having a lower electroconductivity thanthat of the second electroconductive material.

[0021] Furthermore, the first object of the present invention can alsobe achieved by a film with a single layer structure comprising a firstregion extending along a surface of the film and a second regionextending under the first region, the first region comprising aninsulting matrix resin and a first electroconductive material dispersedin the insulating matrix resin, and the second region comprising theinsulating matrix resin and a second electroconductive materialdispersed in the insulating matrix resin, the first electroconductivematerial and the second electroconductive material being of an identicalelectroconductive material, and the first electroconductive materialhaving a larger particle size than that of the second electroconductivematerial.

[0022] Furthermore, the first object of the present invention can alsobe achieved by a film with a single layer structure comprising aninsulting matrix resin and an electroconductive material dispersed inthe form of needles in the insulating matrix resin, with a longitudinalside of the needles of the electroconductive material being oriented inthe direction normal to an external surface of the film.

[0023] In the above film, it is preferable that a cross section of eachof the needles of the electroconductive material, in the directionnormal to the external surface of the film, be in the shape of aquadrilateral with unequal diagonal lines, a longer diagonal line beingoriented in the direction normal to the external surface of the film anda shorter diagonal line being oriented in the direction normal to thelonger diagonal line.

[0024] Furthermore, it is preferable that the above quadrilateral have apair of equal adjacent sides directed to the external surface of thefilm, and a pair of equal adjacent sides directed to a back side of thefilm.

[0025] Furthermore, it is also preferable that the pair of equaladjacent sides of the quadrilateral directed to the external surface ofthe film be shorter than the pair of equal adjacent sides of thequadrilateral directed to the back side of the film.

[0026] The second object of the present invention can be achieved by amethod of producing an endless-belt-shaped film with a single layerstructure comprising an insulting matrix resin and an electroconductivematerial dispersed in the form of particles in the insulating matrixresin, with larger particles of the electroconductive material beingpositioned on the side of an external surface of the endless-belt-shapedfilm, and smaller particles of the electroconductive material beingpositioned on the side of an inner surface of the endless-belt-shapedfilm, comprising the steps of:

[0027] dispersing the electroconductive materials with differentparticles size in a solution of the matrix resin in a solvent to form adispersion of the electroconductive materials in the solution of thematrix resin, and

[0028] subjecting the dispersion to centrifugal molding to localizelarger particles of the electroconductive material on the side of theexternal surface of the endless-belt-shaped film, and to localizesmaller particles of the electroconductive material on the side of theinner surface of the endless-belt-shaped film, with removal of thesolvent therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

[0030]FIG. 1 is a schematic cross-sectional view of a centrifugalmolding cylinder 1 for forming an endless-belt-shaped film serving as anintermediate image transfer belt on a molding surface thereof bycentrifugal molding.

[0031]FIG. 2 is a schematic partial cross-sectional side view of anendless belt film 2-1 prepared in Example 1, using the centrifugalmolding cylinder 1.

[0032]FIGS. 3A to 3C are schematic cross sectional views of needles ofan electroconductive material in the endless belt-shaped film, showingvarieties of the cross-section of the needles.

[0033]FIG. 3D is a side view of crushed pieces of selenium 3 b, showingthe shape thereof, used in Example 2.

[0034]FIG. 4 is a schematic partial cross-sectional view of an endlessbelt film 2-2 prepared in Example 2, using the centrifugal moldingcylinder 1.

[0035]FIG. 5 is a schematic partial cross-sectional view of acomparative endless belt film 2′ prepared in Comparative Example 1,using the centrifugal molding cylinder 1.

[0036]FIG. 6A is a schematic plan view of a face electrode in thearrangement of electrodes in resistivity test specified in JIS K69115.13.1.

[0037]FIG. 6B is a schematic plan view of a back electrode in thearrangement of electrodes in resistivity test specified in JIS K69115.13.1.

[0038]FIG. 7A is a diagram in explanation of the connection ofelectrodes in a volume resistivity test specified in JIS K6911 5.13.1.

[0039]FIG. 7B is a diagram in explanation of the connection ofelectrodes in a surface resistivity test specified in JIS K6911 5.13.1.

[0040]FIG. 8 is a diagram in explanation of an insulation resistancemeasuring apparatus specified in (1.1) in JIS K6911 5.13.1.

[0041]FIG. 9 is a diagram in explanation of a measuring method ofsurface resistivity specified in JIS R 3256 3.

[0042]FIG. 10A is a plan view diagram in explanation of an example of amethod for forming electrodes on a test piece for use in a method ofmeasuring surface resistivity specified in JIS R 3256 3.

[0043]FIG. 10B is a front view diagram in explanation of the example ofthe method for forming electrodes on a test piece specified in FIG. 10A.

[0044]FIG. 11 is a schematic partial cross-sectional view of anotherexample of an intermediate image transfer belt of the present invention.

[0045]FIGS. 12A to 12F are diagrams in explanation of a method ofproducing an intermediate image transfer belt of the present invention.

[0046]FIG. 13A is a schematic plan view of electrodes for measuringsurface resistivity.

[0047]FIG. 13B is a schematic cross-sectional view of the electrodesshown in FIG. 13A.

[0048]FIG. 14 is a schematic diagram of an image formation apparatusprovided with an intermediate image transfer belt of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] In an image formation process using an image transfer belt towhich a toner image is transferred, when the electric resistivity in thedirection of an image transfer surface of the image transfer belt isdifferent from that in the direction of the thickness of the imagetransfer belt, the electric resistivity in the direction of the surfaceof the image transfer belt determines the degree of the disturbance ordeformation of the arrangement of the electric lines of force in thedirection of the surface of the image transfer belt, which electriclines of force are formed between a latent image bearing member whichbears the toner image and the image transfer belt before the transfer ofthe toner image. Furthermore, the electric resistivity in the directionof the thickness of the image transfer belt determines the degree of theeasiness of the flow of electric charges remaining on the image transfersurface of the belt to a non-image transfer surface of the belt, thatis, the easiness of the grounding of the image transfer belt. Therefore,by controlling the electric resistivity in the direction of the imagetransfer surface of the image transfer belt and that in the direction ofthe thickness of the image transfer belt, the deformation of thearrangement of the electric lines of force in the direction of thesurface of the belt, which contribute to the image transfer performance,and the easiness of the grounding to remove the electric chargesremaining on the image transfer surface of the belt can be separatelyadjusted.

[0050] In particular, when the electric resistivity in the direction ofthe image transfer surface of the image transfer belt is greater thanthat in the direction of the thickness of the image transfer belt, theelectric charges on the image transfer surface of the image transferbelt more easily flow in the direction of the thickness of the imagetransfer belt than in the direction of the surface of the image transferbelt, so that the deformation of the arrangement of the electric linesof force in the direction of the image transfer surface of the imagetransfer belt can be reduced, and the grounding of the electric chargeson the image transfer surface of the image transfer belt can befacilitated.

[0051] In a method of forming an endless-belt-shaped film, using acentrifugal molding method, more specifically, in a step of forming theendless-belt-shaped film on a molding surface of a centrifugal moldingcylinder by subjecting a base material for the endless-belt-shaped filmto centrifugal molding, in which base material an electroconductivematerial in the form of needles is dispersed, the needles of theelectroconductive material can be arranged in such a manner that thelongitudinal side of each needle is directed in the direction of thethickness of the belt. In the image transfer belt in which the needlesof the electroconductive material, the resistivity in the direction ofthe surface thereof can be made larger than that in the direction of thethickness thereof.

[0052] In particular, when each needle of the electroconductive materialis made in such a shape that one half or a top portion of the needle isheavier than the other portion of the needle, the needles of theelectroconductive material can be easily arranged in the course of thecentrifugal molding in such a manner that the heavier portion of eachneedle is directed to the image transfer surface of the image transferbelt and the ligher portion of each needle is directed to the back sideof the image transfer belt.

[0053] More specifically, with reference to FIG. 3A, a cross section ofeach of the needles of the electroconductive material in the directionnormal to the external surface of the film, may be in the shape of aquadrilateral with unequal diagonal lines, a longer diagonal line abeing oriented in the direction normal to said external surface of thefilm and a shorter diagonal line b being oriented in the directionnormal to the longer diagonal line a.

[0054] As shown in FIG. 3B, in the above quadrilateral, it is preferablethat a pair of adjacent sides thereof m, m′ directed to the externalsurface of the film be equal (m=m′), and that another pair of adjacentsides n, n′ thereof directed to a back side of the film be also equal(n=n′).

[0055] As shown in FIG. 3C, in the above quadrilateral, it is preferablethat the pair of equal adjacent sides m of the quadrilateral directed tothe external surface of the film are shorter than the pair of equaladjacent sides n of the quadrilateral directed to a back side of thefilm (m<n)

[0056]FIG. 1 is a schematic cross-sectional view of a centrifugalmolding cylinder 1 for forming an endless-belt-shaped film serving as anintermediate image transfer belt by centrifugal molding, which ishereinafter referred to as an endless-belt-shaped intermediate imagetransfer film 2. The endless-belt-shaped intermediate image transferfilm 2 is formed along a molding surface 1 a of the centrifugal moldingcylinder 1. In FIG. 1, an alternate long and short dash line 1 cindicates an axis of rotation of the centrifugal molding cylinder 1 inthe course of the centrifugal molding. The centrifugal molding cylinder1 is made of a metal such as aluminum, with the molding surface 1 abeing subjected to mirror finish with high precision.

[0057] The endless-belt-shaped intermediate image transfer film 2 isformed by centrifugal molding on the molding surface 1 a, usingpolyamide acid or polyamic acid serving as a precursor of polyimide.Polyamide acid has the properties of being converted to a polyimidethrough imide ring closure.

[0058] In the present invention, polyamide acid is used as a startingmaterial for forming the endless-belt-shaped intermediate image transferfilm of which base material is polymide.

[0059] The base material for the endless-belt-shaped intermediate imagetransfer film of the present invention is not limited to theabove-mentioned polyimide, but, for example, the following materials canbe used: polyether sulfone, polycarbonate, polyester, polyarylate,polyphenylene sulfide, polyamide, polysulfone, polyparabanic acid,fluoroplastic, polyamide imide, polyether imide, thermosettingunsaturated polyester, and epoxy thermosetting resin.

[0060] Polyamide acid has the properties of performing imide ringclosure when dissolved in a particular organic solvent with theapplication of heat thereto, or in the presence of a catalyst.

[0061] A solution of polyamide acid for the formation of polyimide canbe obtained by allowing an organic diamine to react with an organictetracarboxylic acid dianhydride in an equimolar ratio in an organicsolvent. In the present invention, a commercially available polyimideprecursor solution (Trademark “TORAYNEECE #3000”, made by TorayIndustries, Inc.) is used as the polyamide acid, and diluted withN,N-dimethyl acetamide (hereinafter referred to as DMAC) appropriately,whereby the polyamide acid solution is prepared.

[0062] The organic solvent for dissolving the polyimide precursor whichis used in the preparation of the endless-belt-shaped intermediate imagetransfer film of the present invention, that is, the solvent fordissolving the starting material for forming the endless-belt-shapedintermediate image transfer film, is not limited to CMAC, but anysolvents that are capable of dissolving the starting material can beemployed. Specific examples of such solvents are γ-butyrolactone,dimethyl formamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone,diethylene glycol dimethyl ether, pyridine, dimethyl sulfone,dichloromethane, trichloro-methane, dioxane, and toluene.

[0063] The concentration of the starting material in the above solventsis not limited to a particular concentration, provided that it ispreferable to appropriately adjust the viscosity of the solution of thestarting material for forming the endless-belt-shaped intermediate imagetransfer film in order to obtain an endless-belt-shaped intermediateimage transfer film with excellent surface properties and with anappropriate thickness with high precision.

[0064] The preparation of the endless-belt-shaped intermediate imagetransfer film 2, which will be hereinafter referred to as the endlessbelt film 2, will now be explained.

[0065] The endless belt film 2 composed of polyamide acid serving as thestarting material, which includes DMAC and an electroconductive materialwhich will be explained later, is formed on the molding surface 1 a ofthe centrifugal molding cylinder 1 by a conventional centrifugal moldingmethod.

[0066] In the centrifugal molding method, the starting material may befirst coated on the molding surface 1 a of the centrifugal moldingcylinder 1, and then the centrifugal molding cylinder 1 is rotated toperform the centrifugal molding. Alternatively, the centrifugal moldingcylinder 1 is first rotated, and then the starting material is injectedthereinto. The latter method is generally called “centrifugal casting”.For producing the endless-belt-shaped film of the present invention,either the former method or the latter method can be equally used.

[0067] In the present invention, the former method was used. Morespecifically, the above-mentioned polyamide acid solution was spraycoated on the molding surface 1 a of the centrifugal molding cylinder 1as it was rotated slowly, and the centrifugal molding cylinder 1 wasthen rotated at a high speed of 1000 rpm around the axis of rotation 1 cas shown in. FIG. 1, whereby an endless belt film 2 with a thickness of50 μm was formed.

[0068] In the course of the centrifugal molding, it is necessary tocontinue the rotation of the centrifugal molding cylinder 1 until DMACis evaporated to some extent from the polyamide acid solution placed onthe molding surface 1 a in order to prevent the deformation of theendless belt film 2 after the centrifugal molding is finished. Therotating time of the centrifugal molding cylinder 1 can be shortened byapplying heat to the polyamide acid solution to promote the evaporationof DMAC as the centrifugal molding cylinder 1 is rotated. In this case,it is preferable that the heat application be controlled so as not toinduce excessive imide ring closure in the polyamide acid.

[0069] The DMAC still remains in the endless belt film 2 thus producedby centrifugal molding. The DMAC is removed therefrom in the course ofthe next drying and curing process.

[0070] The drying and curing process for the endless belt film 2 willnow be explained.

[0071] In order to prepare an intermediate image transfer belt from theendless belt film 2 prepared by the centrifugal molding, it is necessaryto dry the endless belt film 2, and to cure the same with the inducementof the imide ring closure in the polyamide acid contained in the endlessbelt film 2. In other words, it is necessary to convert the endless beltfilm 2 into a cured film made of polyimide which is the base materialthereof. In an example of the intermediate endless image transfer beltof the present invention, the endless belt film 2 was dried at atemperature of 100° C.

[0072] There are two methods for inducing the imide ring closure in thepolyamide acid contained in the endless belt film 2. In one method, acatalyst is used for the inducement of the imide ring closure in thepolyamide acid, and in the other method, the polyamide acid is heated.In the formation of the intermediate image transfer belt of the presentinvention, both methods can be used.

[0073] In the example of the intermediate endless image transfer belt ofthe present invention, the latter method was employed in which the imidering closure was induced in the polyamide acid by heating the polyamideacid. More specifically, the endless belt film 2 on the molding surface1 a was heated to about 250° C. to induce the imide ring closure in thepolyamide acid contained in the endless belt film 2, whereby the endlessbelt film 2 was cured and changed to a cured film.

[0074] As mentioned above, conventionally there is the dilemma that whenthe surface resistivity of the intermediate image transfer belt isincreased in order to reduce the occurrence of the image transfer dust,the deposition of toner on the background of images is increased, whilewhen the surface resistivity of the intermediate image transfer belt isdecreased in order to decrease the deposition of toner on the backgroundof images, the occurrence of the image transfer dust is promoted.

[0075] The inventors of the present invention have discovered that asolution can be given to the above problem by imparting electricanisotropy to the intermediate image transfer belt. The electricanisotropy specifically means that the electric resistivity in thedirection of the intermediate image transfer belt is greater than theelectric resistivity in the direction of the thickness of theintermediate image transfer belt.

[0076] When a toner image formed on a latent electrostatic image bearingmember such as a photoconductor is electrostatically transferred to theintermediate image transfer belt, such electric lines of force thatattract the toner image from the latent electrostatic image bearingmember to the intermediate image transfer belt are formed between thelatent electrostatic image bearing member and the intermediate imagetransfer belt. However when the electric resistance in the direction ofthe intermediate image transfer belt is smaller than the electricresistance in the direction of the thickness of the intermediate imagetransfer belt, the arrangement of the electric lines of force in thedirection toward the surface of the intermediate image transfer belt isdisturbed, so that the transfer position of the toner image on theintermediate image transfer belt is deviated from its right transferposition and therefore the formation of the image transfer dust ispromoted.

[0077] On the image transfer surface of the intermediate image transferbelt, there remain electric charges due to the effects of the tonerdeposited on the image transfer surface of the intermediate imagetransfer belt and also due to the effects of an electric fieldtherearound. The electric charges remaining on the image transfersurface of the intermediate image transfer belt increase the attractionof the toner to the intermediate image transfer belt. Therefore, unlessthe quantity of electric charges of the toner on the intermediate imagetransfer belt is reduced, it is difficult to remove the toner from thesurface of the intermediate image transfer belt, so that the cleaning ofthe intermediate image transfer belt tends to become insufficient andtherefore the deposition of the toner on the background of images iscaused.

[0078] In the present invention, setting the electric resistivity of theintermediate image transfer belt in the direction of the image transfersurface thereof larger than that of the intermediate image transfer beltin the direction of the thickness thereof facilitates the flow ofelectric charges in the direction of the thickness of the intermediateimage transfer belt from the image transfer surface of the intermediateimage transfer belt, whereby the deformation of the arrangement of theabove-mentioned electric lines of force can be reduced, and the electriccharges on the image transfer surface of the intermediate image transferbelt can be easily grounded through the intermediate image transferbelt, and accordingly the formation of the image transfer dust and thedeposition of toner on the background of the images can be significantlyreduced. Such electric charges are generally grounded, for instance,through a rotating roller which is rotated in sliding contact with anon-image-transfer surface of the intermediate image transfer belt.

[0079] Preparation of intermediate image transfer belts with theabove-mentioned electric resistance anisotropy will now be explainedwith reference to the following examples:

EXAMPLE 1

[0080] Commercially available fine carbon fibers cut with a length of 5μm or less serving as an electroconductive material, and polyamide acidwere mixed with a ratio of 0.15:1. This mixture was dispersed in DMACand diluted with DMAC so as to prepare a polyamide acid solution with asolid component ratio of 30%.

[0081] The thus prepared polyamide acid solution was subjected tocentrifugal molding on the molding surface 1 a of the centrifugalmolding cylinder 1. In the course of the centrifugal molding, the cutcarbon fibers in the polyamide acid solution were moved toward themolding surface 1 a, with the longitudinal direction thereof beingdirected so as to be in parallel with the direction of the centrifugalforce in the peripheral radius direction by the centrifugal force of therotating molding surface 1 a of the centrifugal molding cylinder 1.

[0082]FIG. 2 is a schematic partial cross-sectional view of an endlessbelt film 2-1 thus prepared in Example 1, which was formed in thecentrifugal molding cylinder 1.

[0083] As shown in FIG. 2, in the course of the movement of the carbonfibers, carbon fibers 3 a in the endless belt film 2-1 were orientedwith the longitudinal direction thereof being directed in the directionof the thickness of the endless belt film 2-1.

[0084] The thus molded endless belt film 2-1 was dried and cured,whereby an intermediate image transfer belt was prepared.

[0085] The thus prepared intermediate image transfer belt wasincorporated in a commercially available full-color copying machine(Trademark “PRETER”, made by Ricoh Company, Ltd.) and color copies weremade. The result was that high quality color images free from imagetransfer dust and the deposition of toner on the background of imageswere obtained. In this full-color copying machine, each toner colorimage formed on a photoconductor drum was transferred to theintermediate image transfer belt, using electrostatic force, in asuperimposed manner to prepare the full-color image.

[0086] The electric resistance in the direction of an external surfaceof the intermediate image transfer belt prepared in Example 1 measuredabout 10⁹ Ω, while the electric resistance in the direction of athickness of the intermediate image transfer belt measured 5×10⁹ Ω.

[0087] A volume resistivity [Ω·cm] of the intermediate image transferbelt in any examples including this example can be determined bymultiplying the electric resistance [Ω] in the direction of thethickness of the intermediate image transfer belt by the thickness ofthe intermediate image transfer belt, 50 μm.

EXAMPLE 2

[0088] In this example, as the electroconductive material, a materialprepared by crushing spherulites of selenium to a grain size of 1 μm orless was employed.

[0089] Spherulite is a kind of crystalline structure observed in manykinds of polymeric materials. Spherulites are radial aggregates ofspherical polycrystals composed of crystals of ortho-rhombic system orcrystals of monoclinic system. Each crystal in the spherulites grows ina radial direction, so that each crystal is in such a shape that itswidth increases outwards from a central point, or in the shape of awedge. Therefore, each crystal has its center of gravity shifted towardan outer side in the longitudinal direction thereof. In other words,when the spherulites are crushed and separated in the longitudinaldirection of each crystal, wedge-shaped pieces of the electroconductivematerial are formed, having a heavier end portion and a lighter endportion.

[0090] As a matter of fact, it was confirmed that crushed pieces ofselenium 3 b, which were obtained by crushing crystallized selenium to agrain size of 1 μm or less, were in a different shape from that of thecut carbon fibers employed in Example 1, each of which was in the shapeof a whisker. More specifically, FIG. 3 shows the shape of each of thecrushed pieces of selenium 3 b, which is a side view of the crushedpieces of selenium 3 b. As shown in FIG. 3, it was confirmed that eachof the opposite ends of the crushed piece of selenium 3 b has the shapeof a cone by SEM inspection. Each of the spherulites of selenium beforethe spherulites were crushed had a grain size of about 50 μm.

[0091] The crushed pieces of selenium 3 b and polyamide acid were mixedwith a ratio of 0.20:1. This mixture was dispersed in DMAC and dilutedwith DMAC so as to prepare a polyamide acid solution with a solidcomponent ratio of 30%.

[0092] The thus prepared polyamide acid solution was subjected tocentrifugal molding on the molding surface 1 a of the centrifugalmolding cylinder 1. In the course of the centrifugal molding, thecrushed pieces of selenium 3 b in the polyamide acid solution were movedtoward the molding surface 1 a, with the heavier cone-shaped end portionthereof (the cone-shaped end portion of the piece of selenium 3 b on theleft side in FIG. 3) being directed to the molding surface 1 a of thecentrifugal molding cylinder 1, by the centrifugal force of the rotatingmolding surface 1 a of the centrifugal molding cylinder.

[0093]FIG. 4 is a schematic partial cross-sectional view of an endlessbelt film 2-2 thus prepared in Example 2, which was formed in thecentrifugal molding cylinder 1. As shown in FIG. 4, in the course of themovement of the crushed pieces of selenium 3 b, the crushed pieces ofselenium 3 b in the endless belt film 2 were oriented with thelongitudinal direction thereof being directed in the direction of thethickness of the endless belt film 2.

[0094] The movement of such an electroconductive material and theorientation thereof with the longitudinal direction thereof beingdirected in the direction of the thickness of the endless belt film inthe course of the centrifugal molding is not limited to the grains ofthe electroconductive material having such a cone-shaped end portion onthe opposite sides thereof as in the above-mentioned crushed pieces ofselenium 3 b, but the above-mentioned movement can also occur in anygrains of the electroconductive material if each of the grains has aheavier end portion and a lighter end portion in a longitudinaldirection thereof.

[0095] In the case of the endless belt film 2-2 shown in FIG. 4, thegrains of the electroconductive material were more easily arranged so asto be directed in the direction of the thickness of the endless beltfilm 2-2 than in the case of the endless belt film 2-1 shown in FIG. 2in Example 1.

[0096] The thus molded endless belt film 2-2 was dried and cured,whereby an intermediate image transfer belt was prepared.

[0097] The thus prepared intermediate image transfer belt wasincorporated in the same full-color copying machine as employed inExample 1 and color copies were made. The result was that high qualitycolor images free from image transfer dust and the deposition of toneron the background of images were obtained.

[0098] The electric resistance in the direction of the external surfaceof the intermediate image transfer belt prepared in Example 2 measuredabout 10⁹ Ω, while the electric resistance in the direction of thethickness of the intermediate image transfer belt measured 5×10⁹ Ω.

EXAMPLE 3

[0099] In this example, as the electroconductive material, a crushedcarbonized material 3 c was employed, which was prepared by carbonizingspherulites of an organic polymer in an atmosphere of an inert gas, andcrushing the carbonized spherulites to a grain size of 1 μm or less. AnSEM inspection of the thus prepared crushed carbonized material 3 cindicated that pieces of the crushed carbonized material 3 c had thesame shape as that of the pieces of the crushed selenium 3 b employed inExample 2.

[0100] The crushed carbonized material 3 c and polyamide acid were mixedwith a ratio of 0.15:1. This mixture was dispersed in DMAC and dilutedwith DMAC so as to prepare a polyamide acid solution with a solidcomponent ratio of 30%.

[0101] The thus prepared polyamide acid solution was subjected tocentrifugal molding on the molding surface 1 a of the centrifugalmolding cylinder 1. In the course of the centrifugal molding, the grainsof the crushed carbonized material 3 c in the polyamide acid solutionwere moved toward the molding surface 1 a, with the heavier cone-shapedend portion thereof being directed to the molding surface 1 a of thecentrifugal molding cylinder 1 in exactly the same manner as in thepieces of the crushed selenium 3 b employed in Example 2, by thecentrifugal force of the rotating molding surface 1 a of the centrifugalmolding cylinder.

[0102] In the above, the grains of the crushed carbonized materialserving as the electroconductive material were more easily arranged soas to be directed in the direction of the thickness of the endless beltfilm than in the case of the endless belt film shown in FIG. 2 inExample 1.

[0103] The state of the grains of the crushed carbonized material 3 c inthe endless belt film was the same as that of the crushed pieces ofselenium 3 b in the endless belt film 2-2 as shown in FIG. 4, so that afigure showing the state of the grains of the crushed carbonizedmaterial 3 c in the endless belt film is omitted.

[0104] The thus molded endless belt film was dried and cured, whereby anintermediate image transfer belt was prepared.

[0105] The thus prepared intermediate image transfer belt wasincorporated in the same full-color copying machine as employed inExample 1 and color copies were made. The result was that quality colorimages free from image transfer dust and the deposition of toner on thebackground of images were obtained.

[0106] The electric resistance in the direction of the external surfaceof the intermediate image transfer belt prepared in Example 2 measuredabout 10⁹ Ω, while the electric resistance in the direction of thethickness of the intermediate image transfer belt measured 5×10⁹ Ω.

[0107] Conventional intermediate image transfer belts without anisotropywith respect to electric resistivity will now be explained ascomparative examples.

Comparative Example 1

[0108] A commercially available spherical carbon black serving as anelectroconductive material, and polyamide acid were mixed with a ratioof 0.15:1. This mixture was dispersed in DMAC and diluted with DMAC soas to prepare a polyamide acid solution with a solid component ratio of30%.

[0109] The thus prepared polyamide acid solution was subjected tocentrifugal molding on the molding surface 1 a of the centrifugalmolding cylinder 1.

[0110]FIG. 5 is a schematic partial cross-sectional view of acomparative endless belt film 2′ thus prepared in Comparative Example 1,which was formed in the centrifugal molding cylinder 1.

[0111] As shown in FIG. 5, spherical carbon black particles 3 d wereregularly dispersed in the comparative endless belt film 2′.

[0112] The thus molded endless belt film 2′ was dried and cured, wherebya comparative intermediate image transfer belt No. 1 was prepared.

[0113] The thus prepared comparative intermediate image transfer beltNo. 1 was incorporated in the same commercially available full-colorcopying machine as employed in Example 1, and color copies were made.The result was that there was formed no image transfer dust, but thedeposition of toner on the background of images occurred in the obtainedimages.

[0114] The electric resistance in the direction of the external surfaceof the intermediate image transfer belt prepared in Comparative Example1 measured about 10⁹ Ω, while the electric resistance in the directionof the thickness of the intermediate image transfer belt measured 5×10¹³Ω.

Comparative Example 2

[0115] The procedure of preparing the comparative intermediate imagetransfer belt No. 1 in Comparative Example 1 was repeated except thatthe commercially available spherical carbon black serving as theelectroconductive material, and polyamide acid were mixed with a ratioof 0.20:1, whereby a comparative endless belt film was prepared inComparative Example 2. The spherical carbon black particles 3 d weredispersed almost in the same manner in the comparative endless belt film2 as in the comparative endless belt film 2′ in Comparative Example 1,so that a figure showing the comparative endless belt film is omittedhere. However, the dispersing density of the spherical carbon blackparticles 3 d in the comparative endless belt film was greater than thatin the comparative endless belt film 2′ in Comparative Example 1.

[0116] The thus molded endless belt film was dried and cured, whereby acomparative intermediate image transfer belt No. 2 was prepared.

[0117] The thus prepared comparative intermediate image transfer beltNo. 2 was incorporated in the same commercially available full-colorcopying machine as employed in Example 1, and color copies were made.The result was that the deposition of toner on the background of imagesdid not occur in the obtained images, but there was formed imagetransfer dust.

[0118] The electric resistance in the direction of the external surfaceof the intermediate image transfer belt prepared in Comparative Example2 measured about 10⁹Ω, while the electric resistance in the direction ofthe thickness of the intermediate image transfer belt measured 5×10¹¹ Ω.

[0119] The following TABLE 1 shows the characteristics of the electricresistance and the quality of the formed image in each of theabove-mentioned examples and comparative examples: TABLE 1 ElectricState of formed images Electric resistance Deposition resistance (Ω) inthe of toner (Ω) in the direction Image on the direction of transferbackground of surface thickness dust of images Ex. 1 10⁹ 5 × 10⁸ NoneNone Ex. 2 10⁹ 5 × 10⁸ None None Ex. 3 10⁹ 5 × 10⁸ None None Comp. 10⁹ 5× 10¹³ None Found Ex. 1 Comp. 10⁹ 5 × 10¹¹ Found None Ex. 2

[0120] The results shown in TABLE 1 indicates that when the conditionsthat the electric resistance in the direction of the surface of theintermediate image transfer belt is greater than that in the directionof the thickness of the intermediate image transfer belt are satisfied,both the formation of the image transfer dust and the deposition of thetoner on the background of the images can be reduced.

[0121] The measurement of the electric resistance of the intermediateimage transfer belt will now be explained.

[0122] The conversion from surface resistance to volume resistivity andvice versa will now be explained.

[0123] In the case of an isotropic electroconductive thin film with athickness T [cm], a surface resistance ρs [Ω], and a volume resistivityρv [Ω·cm], for instance, with reference to a technical article entitled“Resistivity Correction Coefficient” (published by Mitsubishi Yuka Co.,Ltd.), it is known that the following relationship holds between thethickness T [cm], the surface resistance ρs [Ω], and the volumeresistivity

[0124] ρv [Ω·cm]: ρs=ρv/T

[0125] From the above formula, the surface resistance can be convertedinto the volume resistivity and vice versa.

[0126] In the present invention, the surface resistivity and the volumeresistivity were measured in accordance with the respective proceduresdefined in the Japanese Industrial Standards (JIS K 6911 5.13.1).

[0127] More specifically, the surface resistivity was measured byconnecting an outer ring face electrode and an inner circular disc faceelectrode on a test piece, using an insulating resistance measuringapparatus specified in JIS K 6911 5.12, under the conditions as definedin JIS K 6911 5.13.

[0128] The volume resistivity was measured by connecting a faceelectrode and a back electrode of the test piece, using theabove-mentioned insulating resistance measuring apparatus, under theconditions as specified in JIS K 6911 5.13.

[0129] Other methods as specified in the Japanese Industrial Standards(JIS 3256) can also be used for measuring the surface resistivity andthe volume resistivity.

[0130] The Japanese Industrial Standards (JIS K 6911 5.13.1) are asfollows:

[0131]FIG. 6A is a schematic plan view of a face electrode, and FIG. 6Bis a schematic plan view of a back electrode. In these figures, the unitof the dimensions is mm.

[0132]FIGS. 7A and 7B respectively show an arrangement of the electrodesin a volume resistivity test and an arrangement of the electrodes in asurface resistivity test of the Japanese Industrial Standards (JIS K6911 5.13.1)

[0133] (1) Apparatus

[0134] (1.1) A Conductive Rubber Cut Into the Shape Shown in FIG. 6A andFIG. 6B by Hatching, or a Moisture Permeable Conductive Paint.

[0135] (1.2) A power Source, Insulation Resistance Measuring Instrumentand Switches in (1.1.2) Through (1.1.4) of 5.12.1 of the JapaneseIndustrial Standards (JIS K6911 5.12.1).

[0136] (1.3) Micrometer Calliper

[0137] The micrometer calliper for external measurement specified in JISB 7502 or one with an accuracy at least equivalent.

[0138] (1.4) Vernier Calliper

[0139] The vernier calliper for external measurement specified in JIS B7507 or one with an accuracy at least equivalent.

[0140] (2) Test Piece

[0141] Use a test piece molded into a disc about 100 mm in diameter and2 mm in thickness.

[0142] (3) Preconditioning

[0143] Precondition the test pieces under C−90⁺⁴ ⁻² h/20±2° C./(65±5)%RH.

[0144] (4) Procedure

[0145] Measure the thickness of the test piece finished withconditioning, with the micrometer calliper for external measurementaccurate to 0.01 mm, and press the conductive rubber upon the test piecein the positions shown in FIG. 6A and FIG. 6B to be the electrodes.

[0146] Alternatively, the electrodes may be provided by painting on thetest piece with the moisture permeable conductive paint shown in FIG. 6Aand FIG. 6B. In this case, treat the test piece after painting theelectrodes paying attention so that the moisture permeable conductivepaint does not peal off from the test piece during operation.

[0147] Measure the outside diameter of inner circle of face electrodeand the inside diameter of ring electrode with the vernier calliper tothe nearest 0.02 mm. Make connections shown in FIG. 7A for measuringvolume resistivity and shown in FIG. 7B for measuring surfaceresistivity. Connect this assembly in the position of test piece in thesame circuit as that given in 5.12, charge it for 1 min and measure thevolume resistivity and surface resistivity.

[0148] In the above, carry out the tests under the condition in 5.1 (1)[20±2° C. temperature, (65±5)% relative humidity].

[0149] (5) Calculation

[0150] Calculate volume resistivity and surface resistivity by thefollowing equations:

ρv=(πd ²/4T)×Rv

ρs=(π(D+d)/(D−d)×Rs

[0151] where

[0152] ρv: volume resistivity (MΩcm)

[0153] ρs: surface resistivity (MΩ)

[0154] d: outside diameter of inner circle of face electrode (cm)

[0155] t: thickness of test piece (cm)

[0156] Rv; volume resistance (MΩ) D: inside diameter of ring electrodeon face (cm)

[0157] Rs: surface resistance (MΩ) π: ratio of circle's circumference toits diameter =3.14

[0158] The following is a portion of 5.12.1 of JIS K 6911. FIG. 8 is adiagram of Insulation resistance measuring apparatus specified in (1.1)of JIS K 6911 5.12.1.

[0159] (1.1) Insulation Resistance Measuring Apparatus

[0160] An apparatus consisting of electrodes, power source,galvanometer, universal shunt, switches, etc. as exemplified in FIG. 8.

[0161] (1.1.1) Electrodes

[0162] The brass taper pins of Class B specified in JIS B 1352, with 5mm diameter and free from scars on the surfaces.

[0163] (1.1.2) Power Source

[0164] A dry or storage battery at 500 V d.c. voltage. A power sourcefrom rectified a.c. may be used, provided that it is certain that itkeeps a certain d.c. voltage.

[0165] (1.1.3) Insulation Resistance Measuring Instrument

[0166] (1.1.3.1) Measuring Insulation Resistance of not Less Than 1 Mbut Less Than 10⁶ MΩ (Comparison Method)

[0167] The standard resistance shall be of 1 MΩ manganese or one with anaccuracy at least equivalent, and the universal shunt shall be accurateenough for adjusting the deflection and measuring range of thegalvanometer.

[0168] A galvanometer can measure a resistance of less than 10⁶ MΩwith±10% accuracy, if it is highly sensitive with stable zero point asto show 1 mm deflection at 1 m distance by a current of 10⁻¹⁰ A.

[0169] (1.1.3.2) Measuring Insulation Resistance of not More Than 5 MΩ

[0170] Use the insulation resistance tester specified in JIS C 1302.

[0171] (1.1.3.3) Measuring Insulation Resistance of not Less Than 1 MΩbut Less Than 10⁹ MΩ

[0172] Use an apparatus having a d.c. amplifier calibrated to±10%accuracy.

[0173] (1.1.4) Switches

[0174] Switches properly insulated and protected.

[0175] The following is JIS R 3256 3, in which another method ofmeasuring electric resistivity is specified.

[0176]FIG. 9 is a diagram showing the method of measuring surfaceresistivity.

[0177] 3 Measuring Method at Ordinary Temperature

[0178]3.1 Principle of Measurement

[0179] Apply voltage to the surface of test piece using the measuringcircuit composing of an electrode—test piece system X, d.c. power sourceE, d.c. voltmeter V and ammeter A as shown in FIG. 9 and calculate thesurface resistivity by the formula specified in 3.5 from the value ofsurface electrical resistance obtained by dividing this applied voltageby the current flowing on the surface of test piece.

[0180] As a simplified measuring device for surface resistivity in whichthe equipment shown in FIG. 9 is intergrated into one set, highisolation resistance meters are commercially available.

[0181]3.2 Measuring Condition and Applied Voltage

[0182] The measurement shall be carried out after allowing the testpiece to stand at least for 16 hours in the room conditioned at (20±2)°C. in temperature and (50±5)% in relative humidity.

[0183] The applied voltage to the test piece shall be 1,000 V or lowerand the standard voltage shall be 500 V.

[0184] The duration of voltage application shall be one minute normallyand varied according to quality of the glass substrates.

[0185] 3.3 Preparing Method of Test Piece

[0186] The test piece shall be prepared as follows:

[0187] (a) Shape and Dimension

[0188] The test piece shall be a rectangle one side of which is 50 mm ormore or a disk whose diameter is 50 mm or more.

[0189] (b) Surface Condition of Test Piece

[0190] The test piece of mirror finish or similar surface conditionshall be used.

[0191] (c) Washing and Drying

[0192] The test piece shall be washed at first by rubbing using neutraldetergent followed by rinsing using waterworks and then by ultrasoniccleaning within such solvent as extra pure water, acetone, ethanol, etc.

[0193] Drying may be made by using an oven or by natural drying.

[0194] (d) Methods for Forming Electrode on Test Piece

[0195] The formation of electrode on the test piece shall be done byvaporization or sputtering of conductive material, or the like. Gold,platinum, etc. are used as the conductive material but gold ispreferable for this measuring method.

[0196] In this measuring method high insulation resistance is measured,therefore, it is necessary to form a guard electrode in order to removestray current in the electrode—test piece system.

[0197] (e) Dimension of Electrodes

[0198] The electrodes arranged to form concentric circles as shown inFIG. 10A and FIG. 10B shall be used. In this arrangement, the size ofgap may be adjusted by altering the dimension of the main electrodewithin a range of approximately 26 mm to 36 mm, taking sensitivity ofthe measuring device into account.

[0199]3.4 Measuring Procedure

[0200] (a) Form the electrodes on the test piece in accordance with themethod shown in FIG. 10A and FIG. 10B and measure the diameter D₁ of themain electrode and the internal diameter D₂ of the counter electrode atan accuracy of 0.05 mm using the vernier callipers specified in JIS B7507 or the measuring instruments at least equivalent thereto inaccuracy.

[0201] (b) After drying the test piece for at least two hours atapproximately 120° C., cool it in a desiccator.

[0202] (c) Measure the surface resistivity by the measuring method ofsurface resistivity shown in FIG. 9 under the measuring conditions andapplied voltage given in 3.2. The measuring device should be maintainedin an ultra high resistance measuring box shielded.

[0203]3.5 Calculation and Number of Measurements

[0204] The surface resistivity of the glass substrate shall becalculated by the following formula;

ρs [Ω]=(Dmπ/g)Rs

[0205] where,

[0206] ρs: surface resistivity (Ω)

[0207] Rs: surface electrical resistance (Ω).

[0208] Dm: average diameter (D₁+D₂)/2 (mm)

[0209] D₁: diameter of main electrode (mm)

[0210] D₂; internal diameter of counter electrode (mm)

[0211] g: gap (D₂−D₁)/2 (mm)

[0212] The measurements shall be carried out twice and the average valueof the measurements shall be taken as the value of surface resistivity.

[0213]FIG. 11 is a schematic partial cross-sectional view of anintermediate image transfer belt 11, which is another example of theintermediate image transfer belt of the present invention.

[0214] AS shown in FIG. 11, the intermediate image transfer belt 11comprises a substrate 12 and a plurality of first carbon particles 13and a plurality of second carbon particles 14. The first carbonparticles 13 have a lower electroconductivity than that of the secondcarbon particles 14, and have a larger particle size than that of thesecond carbon particles 14.

[0215] Furthermore, as shown in FIG. 11, the presence of the firstcarbon particles 13 is localized on the side of the external surface ofthe intermediate image transfer belt 11, so that the external surface ofthe intermediate image transfer belt 11 is caused to have highresistivity, which is advantageous to obtain high quality image.Furthermore, the intermediate image transfer belt 11 has a single-layerstructure and therefore does not have such a shortcoming that aconventional two-layer intermediate image transfer belt has, that is, ashortcoming that a surface layer is peeled off a substrate for thesurface layer.

[0216] For instance, a polymer for use in the intermediate imagetransfer belt 11 which is used as an intermediate image transfer mediumfor use in a color copying machine is required to have fire retardance,high strength, and electric stability. As such a polymer, for instance,fluoroplastic and polyimide resin are employed.

[0217] In particular, polyimide is a promising material from itsstrength and triboelectric chargeability. A centrifugal molding methodcan be given as a method of forming an endless-belt-shaped intermediateimage transfer belt, using polyimide resin.

[0218] The polyimide resin serving as a basic material for theintermediate image transfer belt can be synthesized from its precursor,polyamide acid. As mentioned above, polyamide acid has the properties ofbeing changed to polyimide with the occurrence of imide ring closurewith the application of heat or in the presence of a catalyst, and alsoof being soluble in a particular solvent. A dispersion of carbonparticles with different particle sizes in the solution of polyamideacid in a particular solvent is hereinafter referred to as a mixedpolyamide acid solution.

[0219] Carbon can be classified into acetylene black, oil furnace black,thermal black, and channel black. Acetylene black can be obtained bysubjecting acetylene to pyrolysis in a preheated furnace. Oil furnaceblack can be obtained by injecting petroleum into a furnace, subjectingthe petroleum to incomplete combustion with the adjustment of the amountof air to be supplied to form carbon, cooling the thus formed carbon,and collecting the carbon, using a cyclone. Thermal black can beobtained by subjecting a natural gas to alternate heat accumulation andpyrolysis at 200° C. to 1700° C. in a heat accumulation furnace. Channelblack can be obtained by blowing a fire of a natural gas against anarrow iron plate so as to deposit carbon on the iron plate.

[0220] It is unnecessary to select a particular carbon from the abovecarbons for use in the intermediate image transfer belt of the presentinvention. However, it should be avoided that carbon capable ofimparting high electroconductivity even in a small amount to theintermediate image transfer belt, such as acetylene black (made by DenkiKagaku Kogyo Kabushiki Kaisha) and Ketjen Black EC (made by LionCorporation), is localized near the surface of the intermediate imagetransfer belt when it is desired to set the surface of the intermediateimage transfer belt at high surface resistivity.

[0221] The carbon can be dispersed in the organic solvent, usingdispersion means, such as an ultrasonic dispersion means, a ball mill ora sand mill. Generally, the carbon is not directly dispersed in thepolyamide acid solution, but is first dispersed in N-methylpyrrolidone(hereinafter referred to as NMP) to prepare a dispersion of the carbon,and the dispersion of the carbon is then mixed with a polyamide acidsolution.

[0222] For example, in the case where the carbon is dispersed in asolvent, using a sand mill, the particle size of the carbon dispersed inthe solvent changes, depending upon the dispersion time, the amount ofthe medium for the dispersion, the number of revolutions of a disk inthe sand mill, and the viscosity of the carbon-dispersed liquid.Therefore, it is necessary to determine the dispersing conditions inadvance in accordance with the desired particle size of carbon particlesby conducting preliminary experiments.

[0223] Thus, a dispersion in which carbon particles with differentelectroconductivities and with different particle sizes are dispersed isprepared. In this case, the dispersion is conducted in such a mannerthat carbon particles with a lower electroconductivity are adjusted tohave a larger particle size.

[0224] A predetermined amount of the mixed polyamide acid solution,which is a mixture of the solution of polyamide acid and the dispersionof carbon particles, is poured into a centrifugal molding cylinder whileit is rotated slowly, and the rotation speed is gradually increasedthereafter to reach a predetermined rotation speed. The rotation iscontinued at the predetermined rotation speed for a predetermined periodof time.

[0225] By this rotation, the mixed polyamide acid solution is subjectedto cast molding inside the centrifugal molding cylinder and at the sametime, the carbon particles begin to be separated. Carbon particles withlarger particle sizes are selectively moved toward the external surfaceof the intermediate image transfer belt and calized near the externalsurface, changing the distribution state of the carbon particles in themixed polyamide acid solution. However, if this rotation is continuedwith higher rotation speed, or for an extended period of time, evencarbon particles with smaller particle sizes also tend to move towardthe external surface. In the present invention, it must be avoided thatcarbon particles with different particle sizes get together near theexternal surface of the intermediate image transfer belt. Therefore, thenumber of revolutions and the rotating time of the centrifugal moldingcylinder are appropriately set so as to avoid the localization of thecarbon particles with different particle sizes near the external surfaceof the intermediate image transfer belt.

[0226] In the course of the rotation of the centrifugal moldingcylinder, the organic solvent is caused to evaporate from the mixedpolyamide acid solution, and the solidification of the polyamide acidproceeds, whereby a cylindrical film is formed. This evaporation ispreferable because it is done in a heated atmosphere, and therefore iscarried out more effectively and more quickly than the evaporation in anatmosphere at normal temperature. It is preferable that the inside ofthe centrifugal molding cylinder, which is a casting mold, be mirrorfinished with highest precision. The size of the centrifugal moldingcylinder should accord with the size of the cylindrical film to be made.

[0227] In order to make the thus obtained polyamide acid film satisfyvarious characteristics such as heat resistance, resistance tochemicals, and mechanical characteristics required, it is necessary tofurther heat the polyamide acid film to perform the imide ring closure.The imide ring closure is conducted by the application of heat to thepolyamide acid film, with complete elimination of any and all solventsremaining in the polyamide acid film by evaporating the solvents. Inpractice, the imide closure may be carried out by heating the polyamideacid film to a predetermined temperature as it is rotated for apredetermined period of time, right after the casting of the polyamideacid film. Alternatively, the polyamide acid film may be released fromthe centrifugal molding cylinder, and then set in a differentcylindrical mold so as to cover the mold with polyamide acid film, andthen heated by heating means using, for example, hot air, whereby apolyimide film can be obtained.

[0228] The thus obtained polyimide film can be used as it is or after itis worked appropriately, as a functional member for variousapplications.

[0229] When this polyimide film is used as the intermediate imagetransfer belt for use in a full-color copying machine, the film is cutin an appropriate size, and if necessary, a skewing stop member isattached to the opposite end portions thereof.

[0230] The thus obtained intermediate image transfer belt includescarbon particles with different electroconductivities, with the carbonparticles with a lower electroconductivity being localized on the sideof the external surface of the intermediate image transfer belt, so thatthe surface of the intermediate image transfer belt has a higher surfaceresistivity. The thus obtained intermediate image transfer belt 11 doesnot have a conventional two-layer structure composed of a substrate anda surface layer, so that the intermediate image transfer belt does nothave a shortcoming that the surface layer is peeled off the substrate.

[0231]FIGS. 12A to 12F are diagrams in explanation of a method ofproducing the intermediate image transfer belt of the present invention.In order to produce the intermediate image transfer belt, to begin with,as shown in FIG. 12A, the following are prepared: a starting material22, which is a 20 wt. % solution of polyamide acid in NMP, a firstdispersion 23, which is a dispersion of acetylene black (Trademark“Denka Back”, made by Denki Kagaku.Kogyo Kabushiki Kaisha) with aparticle size of 0.07 μm in NMP, prepared using a sand mill, and asecond dispersion 24, which is a dispersion of furnace back (Trademark“Asahi #60”, made by Asahi Carbon Co., Ltd.) with a particle size of 0.2μm in NMP, prepared using a sand mill.

[0232] As shown in FIG. 12B, the starting material 22, the firstdispersion 23 and the second dispersion 24 are mixed to prepare apolyamide acid mixed solution 21. The composition of the polyamide acidmixed solution 21 is such that the solid component content of “DenkBlack” is 6 phr and the solid component of “Asahi #60” is 4 phr withrespect to the solid component of the polyimide.

[0233] As shown in FIG. 12C, the polyamide acid mixed solution 21 isinjected into a centrifugal molding cylinder 20 with an inner diameterof 100 mm and a length of 250 mm through an injection tube 19. Thecentrifugal molding cylinder 20 is rotated at 10 rpm in the direction ofthe arrow 25 when the polyamide acid mixed solution 21 is injectedthereinto. This rotating speed is maintained until the injection of thepolyamide acid molding solution 21 is finished.

[0234] As shown in FIG. 12D, when the injection of the polyamide acidmolding solution 21 has been finished, the number of revolutions of thecentrifugal molding cylinder 20 is increased up to 400 rpm in thedirection of the arrow 26, and thereafter the centrifugal moldingcylinder 20 is gradually heated to 100° C., using a sheet-shaped heater24, and the temperature is maintained. Thus, the solvent is caused toevaporate from a polyamide acid solution layer 21 a formed on an innerperipheral surface of the centrifugal molding cylinder 20. As a matterof course, the centrifugal molding cylinder 20 may be heated by heatingmeans other than the above-mentioned sheet-shaped heater 24, such as aheating furnace.

[0235] When the solvent has been sufficiently caused to evaporate fromthe polyamide acid solution layer 21 a, a polyamide acid belt 21 b isformed. The polyamide acid belt 21 b is removed from the centrifugalmolding cylinder 20, and mounted on a mold 27 for changing the polyamideacid belt 21 b to a polyimide belt as shown in FIG. 12E.

[0236] As shown in FIG. 12F, the mold 27, with the polyamide acid belt21 b being mounted thereon, is placed in a furnace 28 in which thetemperature is maintained at 300° C., and the polyamide acid belt 12 bis heated for 20 minutes, whereby an aromatic polyimide belt isobtained.

[0237] The surface resistivity at a face side and that of a back side ofthe aromatic polyimide belt are measured, using electrodes shown in FIG.13A and FIG. 13B, in accordance with the procedure described in theJapanese Industrial Standards (JIS-K 6911).

[0238] In this measurement, a ring electrode 31 and a cylindricalelectrode 32 are placed concentrically on a test piece 34 as shown inFIG. 13A and FIG. 13B. In the measurement, a grounding electrode 33 isdisposed on a back side of the test piece 33. When a surface resistivitybetween the ring electrode 31 and the cylindrical electrode 32 is Rs,the surface resistivity ρs is:

[0239] ρs=18.85 Rs [Ω]

[0240] The surface resistivity at the face side of the aromaticpolyimide belt, ρsA and that at the back side of the aromatic polyimidebelt, ρsB, measured by the above-mentioned method, are respectively asfollows:

[0241] ρsA=6×10¹⁴ Ω and ρsB=3×10⁷ Ω

[0242] In the method of producing the intermediate image transfer beltof the present invention, carbon particles with different particle sizesare mixed with the base material for forming the substrate of theintermediate image transfer belt, and the mixture is then subjected tothe centrifugal molding, whereby carbon particles with larger particlesizes are localized to a surface side of the intermediate image transferbelt, thus regions with different electroconductivities are formed inthe direction of the thickness of the intermediate image transfer belt.

[0243] Furthermore, when as the carbon particles with larger particlesizes, carbon particles with a lower electroconductivity are selectivelyused, the carbon particles with a lower electroconductivity arelocalized on an outer side in the course of the centrifugal molding, sothat a surface layer with higher resistivity can be formed. Thus,according to the present invention, it is unnecessary to provide aseparate surface layer on a substrate as in the conventional method forproducing the intermediate image transfer belt, so that the number ofproduction steps can be reduced.

[0244] The thus produced intermediate image transfer belt can be used,for example, in an image formation apparatus as shown in FIG. 14.

[0245] The image formation apparatus as shown in FIG. 14 is providedwith a photoconductor drum 41 serving as a chargeable image bearingmember, a charger 42 for charging the photoconductor drum 41, anexposure unit 43 for having the charged photoconductor drum 41 exposedto a light image to form a latent electrostatic image on thephotoconductor drum 41, a development apparatus composed developmentunits 44B for development of black, 44C for development of cyan, 44M fordevelopment of magenta, and 44Y for development of yellow, fordeveloping the latent electrostatic image to toner images, anintermediate image transfer belt 11 to which the toner images developedon the photoconductor drum 41 are transferred therefrom, a cleaning unit45 for cleaning the photoconductor drum 41, a cleaning unit 46 forcleaning the intermediate image transfer belt 11, and an image fixingunit 47 for fixing toner images on an image transfer sheet P to whichthe toner images are secondarily transferred from the intermediate imagetransfer belt 11.

[0246] In the above-mentioned example, the case where two kinds ofcarbon particles 13 and 14 are dispersed in the substrate 11 has beenexplained. However, three or more kinds of carbon particles withdifferent particle sizes may be dispersed in accordance with therequired electroconductivity.

[0247] Japanese Patent Application No. 10-270559 filed Sep. 8, 1998 andJapanese Patent Application No. 11-226277 filed Aug. 10, 1999, arehereby incorporated by reference.

What is claimed is:
 1. A film comprising an insulating matrix resin andan electroconductive material, said electroconductive material beingdispersed in said insulating matrix resin in such a manner that asurface resistivity of A [Ω] of said film and a volume resistivity of B[Ω·cm] of said film in the direction of a thickness thereof normal tothe surface of said film satisfy a relationship of: A>B.
 2. The film asclaimed in claim 1, wherein said electroconductive material comprises afirst electroconductive material and a second electroconductivematerial, each of which has a different resistivity.
 3. The film asclaimed in claim 1, wherein said electroconductive material comprises afirst electroconductive material and a second electroconductivematerial, each of which has a different particle size.
 4. The film asclaimed in claim 1, wherein said electroconductive material is in theshape of needles.
 5. The film as claimed in claim 4, wherein each ofsaid needles of said electroconductive material is in such a shape thatthe thickness thereof is decreased toward opposite end portions thereof.6. The film as claimed in claim 1, wherein said electroconductivematerial comprises carbon.
 7. A film comprising an insulating matrixresin and an electroconductive material, having a thickness of T [cm],said electroconductive material being dispersed in said insulatingmatrix resin in such a manner that a surface resistivity of A [Ω] ofsaid film and a volume resistivity of B Ω·cm] of said film in thedirection of a thickness thereof normal to the surface of said filmsatisfy a relationship of: A [Ω]×T [cm]>B [Ω·cm].
 8. The film as claimedin claim 7, wherein said electroconductive material comprises a firstelectroconductive material and a second electroconductive material, eachof which has a different resistivity.
 9. The film as claimed in claim 7,wherein said electroconductive material comprises a firstelectroconductive material and a second electroconductive material, eachof which has a different particle size.
 10. The film as claimed in claim7, wherein said electroconductive material is in the shape of needles.11. The film as claimed in claim 10, wherein each of said needles ofsaid electroconductive material is in such a shape that the thicknessthereof is decreased toward opposite end portions thereof.
 12. The filmas claimed in claim 7, wherein said electroconductive material comprisescarbon.
 13. A film with a single layer structure comprising a firstregion extending along a surface of said film and a second regionextending under said first region, said first region comprising aninsulting matrix resin and a first electroconductive material dispersedin the form of particles in said insulating matrix resin, and saidsecond region comprising said insulating matrix resin and a secondelectroconductive material dispersed in the form of particles in saidinsulating matrix resin, said first electroconductive material having alower electroconductivity than that of said second electroconductivematerial.
 14. A film with a single layer structure comprising a firstregion extending along a surface of said film and a second regionextending under said first region, said first region comprising aninsulting matrix resin and a first electroconductive material dispersedin said insulating matrix resin, and said second region comprising saidinsulating matrix resin and a second electroconductive materialdispersed in said insulating matrix resin, said first electroconductivematerial and said second electroconductive material being of anidentical electroconductive material, and said first electroconductivematerial having a larger particle size than that of said secondelectroconductive material.
 15. A film with a single layer structurecomprising an insulting matrix resin and an electroconductive materialdispersed in the form of needles in said insulating matrix resin, with alongitudinal side of said needles of said electroconductive materialbeing oriented in the direction normal to an external surface of saidfilm.
 16. The film as claimed in claim 13, wherein a cross section ofeach of the needles of said electroconductive material, in the directionnormal to said external surface of said film, is in the shape of aquadrilateral with unequal diagonal lines, a longer diagonal line beingoriented in the direction normal to said external surface of said filmand a shorter diagonal line being oriented in the direction normal tosaid longer diagonal line.
 17. The film as claimed in claim 16, whereinsaid quadrilateral has a pair of equal adjacent sides directed to saidexternal surface of said film, and a pair of equal adjacent sidesdirected to a back side of said film.
 18. The film as claimed in claim17, wherein said pair of equal adjacent sides of said quadrilateraldirected to said external surface of said film are shorter than saidpair of equal adjacent sides of said quadrilateral directed to a backside of said film.
 19. A method of producing an endless-belt-shaped filmwith a single layer structure comprising an insulting matrix resin andan electroconductive material dispersed in the form of particles in saidinsulating matrix resin, with larger particles of said electroconductivematerial being positioned on the side of an external surface of saidendless-belt-shaped film, and smaller particles of saidelectroconductive material being positioned on the side of an innersurface of said endless-belt-shaped film, comprising the steps of;dispersing said electroconductive materials with different particlessize in a solution of said matrix resin in a solvent to form adispersion of said electroconductive materials in said solution of saidmatrix resin, and subjecting said dispersion to centrifugal molding tolocalize larger particles of said electroconductive material on the sideof the external surface of said endless-belt-shaped film, and tolocalize smaller particles of said electroconductive material on theside of the inner surface of said endless-belt-shaped film, with removalof said solvent therefrom.